In the course of studies on the natural history of Toxoplasma gondii, two isospora-type oocysts recovered from the feces of a stray cat produced tissue cysts when administered orally to mice. One was identical to the T. gondii oocyst and the other one was larger. The T. gondii-like oocyst was found to be a stage of a previously undescribed parasite of cats, which has similarities to T. gondii as well as major differences. The name Hammondia hammondi has been proposed for this parasite. The other oocyst was found to produce Besnoitia-type cysts and cats also appear to be its natural host. Thus it was demonstrated that Besnoitia can be spread by cat feces in the same manner as Toxoplasma, Sarcocystis and Hammondia. (Note: Project has been terminated)